Help with changing shower fixture in a tiled shower

My home was built in 1978. The shower has a hot water and a cold water handle. In trying to buy a new fixture all I can find is a single control. I do not want to change the tile but don't know what to do. I'm hoping to change to one of those large shower heads that just rains down on you. Any suggestions? I believe that I can get at the actual plumbing throught the wall behind the shower.

This has come up here recently...you might want to use th search in the blue bar above. Basically, most valves (but not all) meet today's code requirments. This requires it to be a pressure balanced valve to prevent a water pressure or supply problem from scalding somebody in the shower if, for example, someone flushes a toilet. It is easier to do this in a single handle fixture. There are a few two control fixtures, but they generally cost more. Depending on where you live, if just repairing an existing valve, you don't have to get a permit or update the system to today's code (although it is wise to do it). But, some places require you to do this. You can buy replacement valves that do not meet today's code, but they are getting harder to find. Depending on how far apart the handles are, some companies make remodel plates that are big enough to cover many old handles.